The present invention relates to an apparatus and a method for determining thrust bearing misalignment. More particularly, the present invention relates to a thrust bearing misalignment monitor employing thermal measurements to determine misalignment.
Thrust bearings are employed in many machinery articles to prevent excessive axial motion of rotating shafts. Thrust bearings often include fixed Babbitt metal surfaces which interface with a collar portion on the rotating shaft. Thrust bearings find particular applicability in the turbine portions of large steam turbine-generator combinations which are employed by electric utilities. In these turbines, rotating turbine blades or buckets are disposed between stationary turbine diaphragms with clearances between stationary and rotating parts of only a few thousandths of an inch. The turbine rotors typically rotate at speeds of 1800 rpm or 3600 rpm and have rotor radii of as much as 40 or 50 inches or more. It is thus apparent that the linear velocity at the rotor tips is extremely high and may even be transonic. Because of this high velocity and the close tolerances demanded by efficiency considerations, it is seen that the role of the thrust bearing is crucial in preventing axial motion of the turbine rotor. Moreover, because of the large weight of these turbine rotors, vertically acting gravitational forces tend to make vertical misalignment, between the thrust bearing surface and the rotating thrust bearing collar, a more significant problem than that associated with horizontal misalignment. It thus becomes particularly preferable to have continuous monitoring of the vertical misalignment condition.
Thrust bearings are in general provided with some kind of lubricant, such as oil, from a lubricating system. The lubrication system functions to maintain a lubricant film between the thrust bearing surface and the surface of the collar on the rotating shaft. It is, of course, desirable to maintain a minimum film thickness between these surfaces so as to prevent a "wipe" during which there is metal-to-metal contact. For these reasons it is also desirable to be able to continuously monitor the oil film thickness, or at least ensure that minimum oil film thickness design criteria are not being exceeded.
A further aspect of thrust bearings is also noted herein, particularly with respect to thrust bearings which are typically found in large steam turbines. These bearings are typically divided by radial gaps into a plurality of plates or lands circumferentially arranged so as to interface with an annular portion of the thrust collar on the rotating shaft. These lands form what might be described as a set of thermal islands, since the temperature at the surfaces of the individual lands are at least somewhat independent.
In the past, monitoring of the thrust bearing has been accomplished by employing thermocouples embedded in the lands to measure bearing temperature. However, neither bearing misalignment, minimum film thickness, thrust load nor misalignment moment have been determined solely from observations of thrust bearing temperature. Not only would it be highly desirable to obtain such information, but such information is significantly useful if provided on a continuous basis. In the past, other methods of determining bearing misalignment have necessitated disassembly and inspection of the thrust bearing surface itself.